How wide are air traffice "lanes"?

[StrangerThanFiction]

LSLGuy beat me to the punch, but I’ll post this in the interest of fighting ignorance.

@Colophon

Are pressure-based altimeters really that accurate?

As LSLGuy described, they are accurate, but it’s really not the accuracy that matters, it’s the precision. It doesn’t matter if you’re only 34,500’ above sea level when your altimeter says you’re at 35,000’ - as long as everybody else is also 500’ lower than what their altimeters are indicating.

Related to this, could someone explain how the altimeter calibration works? Are the altimeters regularly updated in flight to account for the changing local pressure? Or (as I dimly recall reading), are the flight levels set to a standard pressure so that above a certain height you ignore the calibration and use the standard pressure?

As short primer for those not in the know:

As you ascend, air pressure decreases due to the decreasing weight of less and less atmosphere above you. Barometric altimeters measure that pressure, and display it as feet above mean sea level. Barometric pressure can also change over distance, due to weather phenomena; these are the big H and L that you see on the weather map. Altimeters are dumb: they can only indicate height above or below a plane of constant pressure, or “pressure level.” Where pressure is low, that pressure level will be closer to sea level. This means that if you fly from an area of high pressure to an area of low pressure without resetting your altimeter, it will show that you are higher than you actually are. This is important when you are in the clouds and relying on your altimeter to tell you how high above the trees you are.

Now, to answer your question, yes, pilots must adjust their altimeter setting from time to time to compensate for nonstandard barometric pressure (and in some cases, temperature). The rule in the US is that you must use an altimeter setting from an airport or weather station within 100 miles of your current location. No big deal for a little Cessna flying 100 mph, he has to check his altimeter setting once an hour. But for a jet covering 100 miles in a little over ten minutes, having to reset three or four altimeters seventy times in one flight would introduce unnecessary opportunity for error. With closure rates between two jets exceeding 1000 mph, any altimetry error presents significant hazard to the safety of flight. To remedy this, as you recall, everybody sets their altimeter to a standard setting once they get above a certain altitude. This altitude is called the transition altitude, and it varies from place to place in the world. In the US it is 18,000 feet. Above this, altitudes are referred to as flight levels, because everybody is actually flying along constant pressure levels rather than at constant heights above sea level. As you are climbing through this altitude, you reset your altimeters to standard: 29.92" Hg, or 1013.2 hPa, depending on which system you like the best. Once you have this set, you are not necessarily flying a constant altitude, but rather along a constant pressure plane. The reason we don’t do this down low is that we need the altimeter to be accurately depicting our height above sea level so we can determine our clearance above the terrain or any obstacles.

And if the latter is the case, how do you transition between the two when coming in to land (at which time I assume you will be given the local pressure at the destination airport so you can recalibrate your altimeter)?

When ATC assigns a descent to an altitude below the transition level, the controller gives the pilot the current local altimeter setting for a station close to the aircraft’s current position. From then on, each time the aircraft is passed from one ATC sector to the next, he is provided with a new local altimeter setting. For airplanes that never climb to the transition altitude, each ATC sector along the route provides a current setting. If the airplane is operating VFR, i.e. not talking to ATC, it is the pilot’s responsibility to maintain a valid altimeter setting, obtained from airports or weather stations along his route of flight.

[Dufus]

OP might enjoy this radar track of FedEx thunderstorm deviations

[Richard_Pearse]

@LSLGuy

Prior to about 15 years ago we used 2,000 foot vertical spacing above 29,000 ft. Precisely because at those high altitudes the pressure difference between 29,000 & 30,000 (or 43,000 & 44,000, etc.) was so slight that the worst case of barely-within-tolerance instruments didn’t provide enough clearance if we tried to operate only 1,000 ft apart. Nor, statistically, was the manual or autopilot tracking of altitude precise enough often enough.

The industry spent a lot of money to upgrade the altimeter & autopilot systems of older planes and a lot of old jets got sent to the desert. Now we can resolve altitude tightly enough to safely operate with 1,000 foot spacing all the way up to 50,000. Not that any airliners get to 50,000, but bizjets do.

For the benefit of the others, airspace above 29000’ where 1000’ vertical separation is used is called RVSM airspace (reduced vertical separation minimum) and not all aircraft got updated. The BAe-146s flown in Australia are not approved for 1000’ separation above 29000’.

It is obvious why when you fly one, the difference between the two main altimeters at high altitude may be up to 250’ and there is a pressure error in the autopilot system that causes the aircraft to deviate from its altitude as air speed changes. As you accelerate in level flight after a climb to cruising altitude it will typically climb about 100’ and if you’re told to slow down by ATC you can expect it to descend a similar amount. It basically flies like a sloppy pilot who thinks altitude clearances are advisory only and so it requires more monitoring and intervention by the pilots than a more modern aircraft would.

That doesn’t prevent them from flying above 29000’ it just means that ATC have to be advised (reminded really) that the aircraft is “negative RVSM” when climbing to a level above 29000’ and that RVSM is not applied between it and other aircraft. If one is flying at or above 29000’ there will be 2000’ separation between it and other aircraft even if the other aircraft is approved for RSVM.

[stanger]

I see it mentioned that the horizontal width of the traffic lanes were usually within 2-5 miles wide or so. I think they might be much less than that.

My house is directly under the northwest approach to the airport in San Antonio. Since I am at the edge of the city, and the airport is probably 15-20 miles away, the planes are not flying very low, but you can usually hear the big ones off and on all day long.

When we still had a satellite dish in the backyard, at least once a night we would lose signal for a few seconds and we could hear an airliner overhead coming into town. Some nights this would happen a couple of times.

While there were quite a few flights coming in overhead each night, it always amazed me that a good number of them would fly directly between my satellite dish and the satellite and disrupt my signal. That’s a pretty narrow horizontal path - the width of the span of the plane.

[Richard_Pearse]

@stanger

While there were quite a few flights coming in overhead each night, it always amazed me that a good number of them would fly directly between my satellite dish and the satellite and disrupt my signal. That’s a pretty narrow horizontal path - the width of the span of the plane.

That 2 - 5 nm figure refers to the safety margin given to the airways, the actual navigation systems of the aircraft are accurate to within a few metres most of the time.